Patch type self-locking fasteners in which a plastic patch is adherently bonded to the threads of the fastener are now being used in a wide variety of applications. For example, patch type fasteners are widely used in the aircraft industry and also the ophthalmic industry.
Patch type fasteners are particularly useful in applications where, through vibration or the application of recurrent forces, there is a tendency for the fastener elements to become unthreaded. Where this occurs, the structural members which are held together by the fastener elements may then become separated. In use applications, such as in an aircraft structure, the separation of structural members could be very harmful and could seriously affect the capability of the entire aircraft.
In a patch type self-locking fastener, one of the fastener elements, such as an externally threaded bolt, has a plastic patch which is adherently bonded to a portion of the screw threads of the bolt. When the bolt is then inserted into a complementary female element, the engagement of the plastic patch with the internal threads of the female element produces an increase in the frictional force between the male and the female elements. When the elements are in threaded engagement, the elements may, then, be subjected to vibrational or otherwise recurring forces without loosening of the threaded engagement between the fastener members. If it is desired to disengage the fastener elements, this can be accomplished by applying a torque to the male or female fastener elements with a suitable hand tool. Since the plastic patch is adherently bonded to the threads of the self-locking fastener, the fastener member may be reused a number of times without adversely affecting the bond of the patch to the fastener threads and the performance of the fastener element in providing a firm locking connection with a complementary threaded member.
With the wide usage of self-locking patch type fasteners, it would be desirable to have a more efficient means of producing patch type fasteners. At present, fasteners of this type may be produced by several means as, for example, those illustrated in the Preziosi patent, U.S. Pat. No. 3,294,139; or in the Kull patent, U.S. Pat. No. 3,417,427. As disclosed in the Preziosi patent, a self-locking fastener may be formed by spraying a finely divided plastic onto the fastener element and then heating the fastener element to melt the plastic so that it will flow into contact with the fastener threads. As disclosed in the Kull patent, a patch type fastener may also be formed by forcing a portion of plastic ribbon into intimate contact with the threads of a heated fastener through use of a reciprocating punch which is forceably moved against the fastener threads so as to press the plastic ribbon into the threads.
In manufacturing patch type fasteners, it is essential that the manufacturing procedure be capable of producing fasteners with uniform characteristics. Patch type fasteners are sold according to specifications stating torque required to unthread the fastener members and also the reusability of the fastener, i.e., the number of times the fastener can be untorqued from a complementary member and retorqued, etc. while providing firm engagement with the complementary member. Additionally, of course, the patch type fastener must meet the specifications required of any fastener, such as diameter, construction material, type of thread, etc. The imposition of the additional requirements on a patch type fastener which are related to the nature of the patch itself and the sufficiency of the bond between the patch and the fastener threads, make it necessary that the bond between the patch and the fastener threads be uniform and that the size of the patch and its location on the threads be relatively uniform as between individual fasteners.
In general, the formation of a patch type fastener through application of a finely divided plastic to the fastener threads, as in the Preziosi patent, is not satisfactory due to the nonuniformity of fasteners produced by this procedure. This results primarily from difficulties in metering and applying a powdered plastic to the threads of fasteners in a reproducible manner such that the fasteners have substantially identical characteristics.
The apparatus and procedure of the Kull patent also has drawbacks. In the Kull procedure, a plastic ribbon is forced into the fastener threads by movement of a punch against the fastener threads. By, thus, forcing the plastic ribbon into the fastener threads, the plastic may not bond adequately to the threads, particularly if there are surface imperfections in the surfaces of the thread flanks. Also, since the punch is reciprocated relative to the fastener threads, there is considerable waste movement in the movement of the punch.
Reciprocatory movement is less efficient than other types of movement in a mass production machine since the reciprocating part must accelerate in a particular direction from rest, then come to a complete stop, and then accelerate in the opposite direction from rest. As applied to the Kull machine, the constant reversal of direction by the punch during operation of the machine severely limits its production capabilities.
In view of the drawbacks of previous procedures for the manufacture of self-locking patch type fasteners, it would be desirable to have an apparatus and method which would mass produce patch type fasteners at a higher production rate, while at the same time, producing fasteners having uniform characteristics. Such an apparatus and method would make it easier to supply patch type fasteners to the ever-increasing numbers of users of these products. It would also permit economies in the manufacturing of the fasteners.